Speaker 1: Hey, or hey, I have a really practical question for you. Well,
Speaker 1: you know I'm an engineer, so practical is my middle name.
Speaker 1: All right, Well, here's the question. Who would you trust
Speaker 1: more to build a spare room off of your house?
Speaker 1: A physicist, board engineer. I think you know the answer
Speaker 1: to that, Daniel, Not to physicists. All right, but tell
Speaker 1: me why. You know. Physicists are awesome, but I wouldn't
Speaker 1: say they're very precise. You know, they approximate everything. You know,
Speaker 1: everything's like plus or minus a galactic idea. So you
Speaker 1: want to know how big this bedroom is going to
Speaker 1: be in advance, for example. I just don't want like
Speaker 1: a spherical room or like a quantum on certain room.
Speaker 1: You know, I want to know where I'm sluting. Well,
Speaker 1: you might appreciate a surprise, but I think that's fair,
Speaker 1: that's reasonable. But you know that sometimes physicists can actually
Speaker 1: get really hardcore about the detail. You're still not building
Speaker 1: my spare room. I am more hammy cartoonists and the
Speaker 1: creator of PhD comics. Hi, I'm Daniel. I'm a particle
Speaker 1: of physicist and you definitely don't want me building your
Speaker 1: spare room. Is that because of you or because you're
Speaker 1: a physicist, or is it all packaged together. You know,
Speaker 1: there's a whole spectrum of physicists. There's the kind that
Speaker 1: really likes to build stuff, crawl around on the detector
Speaker 1: with a hammer and a wrench and and get dirty.
Speaker 1: And then there's the kind of like to sit in
Speaker 1: front of a laptop and analyze data and think about statistics.
Speaker 1: And I'm definitely more of that second kind. Well, you
Speaker 1: can check out my house in your laptop and think
Speaker 1: about it for a long time. I'll build you a
Speaker 1: virtual spare room. There you go. I'll program another spare
Speaker 1: room in the simulation of your universe. It sometimes exists
Speaker 1: in this universe and sometimes it doesn't. It's theoretical. That's right.
Speaker 1: Just step through this black hole into your new spare
Speaker 1: rooms and cozy but welcome to our podcast Daniel and
Speaker 1: Jorge Explain the Universe, a production of our Heart Radio
Speaker 1: in which we talk about all the amazing things that
Speaker 1: are happening in this universe, all the things we'd like
Speaker 1: to understand from the very very large, very very dense,
Speaker 1: all the way down to the very very small and
Speaker 1: the very very weird. Yeah, because the universe has a
Speaker 1: lot of amazing things and all kinds of skills, you know,
Speaker 1: galactic cosmological skills, But there are also amazing things happening
Speaker 1: at the smallest scales of reality and nature. That's right,
Speaker 1: and these really tiny things they give us an amazing opportunity.
Speaker 1: They let us test our understanding and not just do
Speaker 1: we mostly get things right, They let us really push
Speaker 1: our limit to understand exactly how these things are working.
Speaker 1: Do our models predict what's happening or is there something
Speaker 1: a little bit wrong? And that kind of raises a
Speaker 1: question of how well do we know what's happening at
Speaker 1: these really tiny scales, Like we can measure distances from
Speaker 1: here to the moon, for example, or here to maybe
Speaker 1: the next star, But how do you measure things that
Speaker 1: are that small? And how do you know if you're right?
Speaker 1: It definitely takes a certain skill. You have to come
Speaker 1: up experimentally with really clever devices, things that isolate individual particles,
Speaker 1: or get a bunch of particles and get them all
Speaker 1: aligned and then separate them from any other effect. It's
Speaker 1: a really particular skill in science to devise an experiment
Speaker 1: that forces nature to reveal something for you that pushes
Speaker 1: everything else away. Like we learned about the Lego experiment
Speaker 1: that measures gravitational waves. There's a huge amount of cleverness
Speaker 1: involved in isolating those things so you can see tiny
Speaker 1: little wiggles in space. Well, this is sort of experimental cleverness,
Speaker 1: and when you deal with particles, you need this sort
Speaker 1: of the same kind of skill. You need to set
Speaker 1: the universe up in a way that it has to
Speaker 1: reveal to you very precisely the answer to your question. Right,
Speaker 1: But sometimes a problem, right, Daniel, is that you measure
Speaker 1: something and you don't get what you expect. You measure something,
Speaker 1: you think it's going to be this big or this
Speaker 1: long or this heavy, and then when you measure in reality,
Speaker 1: it's different. Yeah, well that's not a problem. That's fantastic,
Speaker 1: that's exciting. That's an opportunity, you know, Like, yeah, because
Speaker 1: we have two branches of our work. We have the
Speaker 1: experimental side that's going out and doing stuff and measuring
Speaker 1: things and answering questions, you know, sort of asking the universe,
Speaker 1: what do we conclude from that? How do we interpret
Speaker 1: that and build a model of the universe in our heads,
Speaker 1: and then turn around and predict future measurements and when
Speaker 1: those predictions disagree with the things we observe. That gives
Speaker 1: us an opportunity to update that model, to say, oh,
Speaker 1: something was wrong. There's a new particle, or this particle
Speaker 1: works differently, or black holes are actually bigger than we thought.
Speaker 1: Those are the moments when we learn. So when theory
Speaker 1: and experiment disagree, I smell opportunity. But how do you
Speaker 1: know who's right. You never do, And usually they're both wrong,
Speaker 1: and they're both wrong in different ways because there are
Speaker 1: very very different challenges, you know, split the difference. Calculating
Speaker 1: something theoretically has challenges of computing time and getting minus
Speaker 1: signs right and sort of organized in your mind, and
Speaker 1: and getting answers experimentally has all sorts of different challenges,
Speaker 1: making things clean, making them distinct, getting a big sample
Speaker 1: of something, getting enough material, you know, we're sometimes just
Speaker 1: getting enough money to build the device that you need.
Speaker 1: That's the hard part. But there is one of these
Speaker 1: big mysteries in nature that it has to do with
Speaker 1: a weird kind of discrepancy between what the theory predicts
Speaker 1: and what we actually measure. So today on the podcast,
Speaker 1: we'll be talking about mystery of the muance magnetic moment.
Speaker 1: That sounds marvelous and magnificent. It is one of the
Speaker 1: most amazing and marvelous moments in magnetic field history. You know,
Speaker 1: it's an opportunity for physics to learn something because it's
Speaker 1: something that we know how to calculate very very very precisely.
Speaker 1: You know, if you want to find out what's wrong
Speaker 1: with your theory, you need to find something that you
Speaker 1: can predict very accurately and then measure you're very very accurately,
Speaker 1: so you can compare the two and that tells you
Speaker 1: if your theory is right or and or if you're
Speaker 1: measuring device is working right and not just giving you
Speaker 1: weird things. Yeah, and if you hope that your experiment
Speaker 1: is correct, then you know, if you see a discrepancy,
Speaker 1: tells you that your theory is wrong. And sometimes we
Speaker 1: do this as a way to detect the presence of
Speaker 1: new particles or you know, just to see if anything
Speaker 1: is right, because some of these calculations are very very sensitive,
Speaker 1: so it's a very good way to tell whether there's
Speaker 1: anything missing in your ideas. You know, it's sort of
Speaker 1: like if you walk around your house and you could
Speaker 1: take a really precise measurement, you could see where everything was,
Speaker 1: and you compared that to the drawings you have of
Speaker 1: your house. Right, that would tell you, like, you know,
Speaker 1: whether your house is well described by your idea of it,
Speaker 1: or whether it was a mistake to hire you to
Speaker 1: clearly actually probably not. How are you to measure it
Speaker 1: also build it? I would say, Wow, this is perfect work.
Speaker 1: You should pay your contract or double. I guess that's
Speaker 1: a big question. And the question is how good or
Speaker 1: physics isn't measuring things? And so we were wondering, we
Speaker 1: were curious about how many people out there. So if
Speaker 1: I've thought about how good our measurements of the universe are,
Speaker 1: and in particular, what's the most precisely measured quantity in physics? So,
Speaker 1: as usual Daniel went out there into the wilds of
Speaker 1: the Internet to ask people what's the most precisely measured
Speaker 1: quantity in physics? That's right, And if you're interested in
Speaker 1: answering random internet questions without any preparation, please right to
Speaker 1: me at feedback at Daniel and Jorge dot com and
Speaker 1: I'll send you some questions to answer. So think about
Speaker 1: it for a second. If you were asked, what's the
Speaker 1: most precisely measured thing in physics. What would you answer?
Speaker 1: Here's what people had to say. My best guess is
Speaker 1: increments of time, accusing an atomic clock. I don't know.
Speaker 1: I don't know, Sorry that I don't know. Is this
Speaker 1: something to do with the plank Clint? Maybe aliens? I'm
Speaker 1: not sure what you mean about quantity, whether that be
Speaker 1: amount of things, how many bananas it takes to create
Speaker 1: a black hole. I'm going to guess it's mass. Maybe
Speaker 1: I like this question. Well, I think first we have
Speaker 1: to define what do we mean by precisely? Maybe temperature
Speaker 1: mass would be the most precisely measured quantity in physics
Speaker 1: because it holds a tangible value. I reckon they can
Speaker 1: measure pretty small, like maybe an adam. All ride a
Speaker 1: broad range of answers, from aliens to bananas to the
Speaker 1: plank scale. I feel like our audience is very much
Speaker 1: in tune with what we cover here in the podcast. Yeah,
Speaker 1: these are great answers, and I have to confess I
Speaker 1: think that some of these answers make me rethink how
Speaker 1: I should have asked this question, because I asked, what's
Speaker 1: the most precisely measured quantity in physics? Like you go out,
Speaker 1: you do an experiment, you measure something. But I think
Speaker 1: really the question we should have asked is what's the
Speaker 1: most precisely calculated quantity in physics? And there's a difference.
Speaker 1: For example, the atomic clock answer is a really good one.
Speaker 1: You know, atomic clocks are precise us too, like one
Speaker 1: part and tend to the sixteen. You know, it takes
Speaker 1: like ten to the sixteen seconds before they're off by
Speaker 1: one second. So you would agree with a lot of
Speaker 1: these alien, Yeah, some of these are really very accurate.
Speaker 1: And for example, Lego, like we mentioned before, gravitational waves.
Speaker 1: To detect gravitational waves, they have to measure, you know,
Speaker 1: the change in length of something by one part in
Speaker 1: tend to the twenty or tend to the twenty one,
Speaker 1: which is really incredibly precise experimentally. But what I was
Speaker 1: going for was a question of, like, what's the most
Speaker 1: precise test we have of our theories, which requires not
Speaker 1: just a really precise experiment, but also a really precise prediction. Oh,
Speaker 1: I see, like what's the most precise that physicists have
Speaker 1: been right about stuff? Is that kind of what you mean? Yeah,
Speaker 1: because for gravitational waves, for example, that's a very precise
Speaker 1: experimental measurement but we didn't know in advance how big
Speaker 1: it would be, and we don't know necessarily how big
Speaker 1: those should be. It depends on the size of the
Speaker 1: black holes that are the same with the tomic clocks.
Speaker 1: We can't calculate the those things as well as we
Speaker 1: can measure them. In order to get some insight into
Speaker 1: the universe, you need something where you can calculate it
Speaker 1: really well and you can predict it really precisely. Interesting,
Speaker 1: so you're kind of talking about like, according to the
Speaker 1: laws of physics, we think that dis quantity should be this,
Speaker 1: and then how well does it match with what we
Speaker 1: actually measure of it exactly? Because it's those discrepancies we
Speaker 1: need to learn something. It doesn't matter if you measure
Speaker 1: the length of your house to one picometer, because we
Speaker 1: don't know how big your house should be and doesn't
Speaker 1: really tell us anything about the universe. But if you
Speaker 1: measure something really precisely that we can also predict that
Speaker 1: we can calculate that has to be a certain value
Speaker 1: because of our understanding of physics. Then measuring it and
Speaker 1: finding out it's something else gives you a clue that
Speaker 1: something is wrong about our model. Right, Well, it seems
Speaker 1: like there's one such thing that we're trying to predict
Speaker 1: and measure at the same time, and that there's a
Speaker 1: big mystery about why those two things don't match. And
Speaker 1: that's the magnetic moment of muance, which is a great
Speaker 1: alliteration there. I'm so pleased to have some positive feedback
Speaker 1: for a name in particle physics from you. That's a high,
Speaker 1: high standard for your poetic writing. Here, the mystery of
Speaker 1: the muon magnetic moment. Yes, it's really marvelous, all right.
Speaker 1: So this is a quantity that we have predicted using
Speaker 1: theory and that we've measured using big machines. But those
Speaker 1: two things don't match. That's right, all right, So let's
Speaker 1: get into it, Daniel. Let's start with the first m
Speaker 1: What is the magnetic moment. So when you think about particles,
Speaker 1: remember we like to think of them as little dots
Speaker 1: in space that have labels, and those labels can be
Speaker 1: like what's the spin or what's the charge or how
Speaker 1: much mass do they have. We don't think of particles
Speaker 1: is like little physical balls that actually do these things.
Speaker 1: They're weird quantum objects and they have these labels. And
Speaker 1: so this is one of the labels of a particle.
Speaker 1: But it's a little weird because it's not like a
Speaker 1: direct label. It's not like something you can put right
Speaker 1: on the particle. Because particles they don't have a magnetic charge.
Speaker 1: They have electric charge. That's how they feel electric fields.
Speaker 1: But as we talked about on the podcast before, there
Speaker 1: are no particles that just have like a north or
Speaker 1: a south magnetic charge on their own. See, they have
Speaker 1: an electric charge, but they don't have like a pole
Speaker 1: like you say, like a magnet, like a north and south.
Speaker 1: That's right, they don't have just a north and just
Speaker 1: a south. What they have is this weird magnetic field.
Speaker 1: It's a dipole to have a north and a south,
Speaker 1: just like every magnet we've ever discovered has a north
Speaker 1: and a south. And that comes from the combination of
Speaker 1: having charge and having spin because charge and spin together
Speaker 1: gives you some sort of magnetic interaction. Okay, so particles
Speaker 1: have spin and charge and together they have a like
Speaker 1: a pole, like a magnet, little magnet inside of it. Yeah,
Speaker 1: and that's what we call the magnetic moment. It's the
Speaker 1: part of the muan that is affected by a magnetic field.
Speaker 1: And you know, fundamentally it comes from having charge and
Speaker 1: from spinning. And that's because it has a magnetic moment.
Speaker 1: It doesn't have a magnetic charge. It's not a north
Speaker 1: or a south, but it is affected by the magnetic field.
Speaker 1: And that's what we mean when we say the magnetic
Speaker 1: moment of the muan. How a muan is affected by
Speaker 1: a magnet. It's not the moment for like an electron
Speaker 1: looks at a positron and they feel that attraction towards. No,
Speaker 1: it's not. It's not a dramatic moment. It's not something
Speaker 1: exists in like theory of screenplays or anything like that,
Speaker 1: unless you're writing a movie about particles, in which case
Speaker 1: there probably is an electrifying moment for the muan. Wow,
Speaker 1: you would totally watch that movie. Um, I totally have
Speaker 1: that movie script already in a drawer in my house.
Speaker 1: It's been sent to several Hollywood agents, but nobody seems
Speaker 1: to be writing. Consider this podcast my pitch for this project.
Speaker 1: I would definitely watch that movie, but I have not
Speaker 1: yet written the script anyway. So we're interested in you know,
Speaker 1: what happens when you put a magnetic field on a muan.
Speaker 1: And this is something we can measure because we can
Speaker 1: do that experiment, and it's also something we can calculate,
Speaker 1: and it turns out to be really sensitive to exactly
Speaker 1: what's happening into some other big questions about how particles
Speaker 1: work well, And maybe let's go back a step and
Speaker 1: cover the other m which is the muon. So muan
Speaker 1: is like an electron, Is it like a quirk? Yeah,
Speaker 1: So we are made out of quirks and electrons. Right,
Speaker 1: We have quirks that make up the protons and neutrons
Speaker 1: inside our atom, and then we have electrons whizzing around them.
Speaker 1: But each of those particles have other copies. There are
Speaker 1: other kinds of quirks, and there are also other kinds
Speaker 1: of electrons. So there's a heavier version of the electron.
Speaker 1: We call that a muan, exactly the same as the electron,
Speaker 1: except has a lot more mass. And there's another one
Speaker 1: even called the towel. So the electron has these two cousins,
Speaker 1: the muon and the towel that have all the same
Speaker 1: interactions and all the same properties, like the same charge,
Speaker 1: the same spin, but just heavier match, just heavier mass. Yeah,
Speaker 1: and it's weird. We don't know why they exist, Like
Speaker 1: why do we have the muan. Why do we have
Speaker 1: the tow Why does the electron have two cousins and
Speaker 1: not nine cousins or seventeen cousins or any cousins like
Speaker 1: my cousin? Are they good for anything? I'm not going
Speaker 1: to get in the middle of that family dispute. We
Speaker 1: have cousins, so we want specified which one I'm talking about.
Speaker 1: But what I guess what I mean is like, is
Speaker 1: it good for anything? Like does it form part of
Speaker 1: you know, can you make an atom out of them?
Speaker 1: Or do we just know them kind of theoretically, or
Speaker 1: we know that they formed, but then they disappear quickly.
Speaker 1: That's right. They're not stable, so you can form atoms
Speaker 1: out of them. You can take a proton and put
Speaker 1: a mu on around it and form a bound state.
Speaker 1: But the muan lasts for you know, a few microseconds.
Speaker 1: Remember that heavy particles don't survive very long in the universe. Actually,
Speaker 1: in its reference frame, if you were riding on the
Speaker 1: back of a muan, you'd see that it lasts a
Speaker 1: few microseconds. But because they move so fast, their clocks
Speaker 1: are slowed down. So as we watch the muan, we
Speaker 1: see them live their three microsecond lifetime. Over a longer
Speaker 1: period because of time dilation, so they don't last terribly long.
Speaker 1: It's still you know, seconds or minutes. But muans don't
Speaker 1: last in our universe because they're heavy. They effectively turn
Speaker 1: into electrons. All right, So there's a big mystery regarding
Speaker 1: the magnetic moment of the eu on. So let's get
Speaker 1: into the theory and the experiment and talk about what
Speaker 1: it means. But first let's take a quick break a right, Daniel,
Speaker 1: we're talking about the magnificent mystery of the marvelous muon
Speaker 1: magnetic moment. Momentarily, hi rolls in your mouth. It's delicious. Um,
Speaker 1: So yeah, so tell me about this mystery. So we
Speaker 1: know about the muan and you're saying that we can
Speaker 1: the theory predicts its magnetic moment. How can the theory
Speaker 1: predict something like that, Well, we think about it in
Speaker 1: terms of particles, right, We're talking about how the muon
Speaker 1: is affected by a magnetic field. But a magnetic field
Speaker 1: we know is really carried by photons. Like when things
Speaker 1: interact electromagnetically, we can imagine that as being done by
Speaker 1: photons moving through space carrying information. Remember, every force that
Speaker 1: we think about electromagnetism, the strong force the weak force
Speaker 1: has these particles to sort of do its job, and
Speaker 1: in the case of the electromagnetic interaction, it's the photon.
Speaker 1: So when you think about how a muon is affected
Speaker 1: by a magnetic field, really on the sort of particle level,
Speaker 1: what you're thinking about is what happens when a photon
Speaker 1: hits a muon, or how does a photon interact with
Speaker 1: a muon. That's sort of like the basic tinker toy
Speaker 1: element of particle physics that lets muans be affected by
Speaker 1: magnetic fields, right, because magnetic fields are transmitted by photons. Yeah,
Speaker 1: magnetic fields are basically photons. We can think about like
Speaker 1: our fields particles or particles fields, but they're very tightly connected.
Speaker 1: So like if I throw a muon at a bunch
Speaker 1: of magnets and it curves one way, it's not because
Speaker 1: it's something in it. It's because it's like hitting and
Speaker 1: interacting with photons. Yeah, exactly, it's getting bent by the
Speaker 1: magnetic field. And very natural way to think about that
Speaker 1: is in terms of photons being generated by you know
Speaker 1: whatever where the source of your magnetic field is and
Speaker 1: pushing the muan. All right, So then we think of
Speaker 1: its interactions is hitting photons, and so how does that
Speaker 1: help us predict its magnetic moment. Well, it's fascinating because
Speaker 1: there's a whole bunch of different ways that a photon
Speaker 1: can hit that mu on. Like the simplest thing is
Speaker 1: photon hits the muon and bounces off, right, So you
Speaker 1: have photon muan interaction very simple, Like in your mind,
Speaker 1: you have a couple just little lines of particles that
Speaker 1: intersect and then they go their separate ways. That's the
Speaker 1: simplest thing, and you can use that to calculate, all right,
Speaker 1: what's the strength of the magnetic moment of the muan?
Speaker 1: And if you did that calculation, you get a pretty
Speaker 1: simple answer. This was done first by a guy named
Speaker 1: Julian Schwinger, and he was so proud of this calculation
Speaker 1: that he actually had this number. It's alpha and the
Speaker 1: fine structure constant over to pie. He put this number
Speaker 1: on his tombstone. He's like, don't forget I came up
Speaker 1: with this. Seriously, it's like it's a beautiful calculation. He
Speaker 1: was so proud. This guy did a huge amount of
Speaker 1: physics in his lifetime. He's basically the person who proved
Speaker 1: that Fynman's theory of quantum electro dynamics actually work. Finding
Speaker 1: like sketched a bunch of doodles and had a few ideas,
Speaker 1: but never like actually made it work. And Julian Schwinger
Speaker 1: was like, all right, let's do all the calculations and
Speaker 1: see if this is right. But that benefit in his
Speaker 1: tool stone, I guess this is a really succinct way
Speaker 1: to just sort of like sum up the guy's life. Anyway,
Speaker 1: the point is that there are other things the photon
Speaker 1: can do also. It doesn't just have to bounce off
Speaker 1: the muan on its way there. It could like split
Speaker 1: into an electron and positron and then convert back into
Speaker 1: a photon and then go off. Or it can emit
Speaker 1: a particle and then reabsorb that first particle. So if
Speaker 1: you'd like drawing these Fineman diagrams these ideas for how
Speaker 1: this happens, all you have to do is add a
Speaker 1: couple more lines, and all these things describe totally valid
Speaker 1: things the photon could do as it interacts with the muon,
Speaker 1: and those change effectively the muan's magnetic moment. See, so
Speaker 1: it's kind of like the muon doesn't really have a
Speaker 1: magnetic moment. How does it interact with a photon? Its
Speaker 1: interaction with the photon is essentially what determines how it
Speaker 1: reacts to magnetic fields, which is its magnetic moment. And
Speaker 1: photons are crazy, they're like always turning into other stuff
Speaker 1: and spewing off particles and reabsorbing them. And the real
Speaker 1: actual thing that happens between a mean and a photon
Speaker 1: is some some of all those things, all those things
Speaker 1: mixed together, which you can and that you can predict
Speaker 1: with the theory. Like your theory, you can like write
Speaker 1: this down in a piece of paper, like what happens
Speaker 1: if a photon hits the muan, and you can in
Speaker 1: a piece of paper you can work out how that
Speaker 1: muan should bend its path or how we should get deflected.
Speaker 1: And then you can say, well, what if it was
Speaker 1: a little bit more complicated, what if it also emitted
Speaker 1: another particle at the same time, Then it would change
Speaker 1: your calculation. And you know, as you make these things
Speaker 1: more complex, there are more and more possibilities, so it
Speaker 1: becomes very challenging theoretically to account for all the different things.
Speaker 1: But that's also gives you an opportunity because if there
Speaker 1: are crazy particles out there that you would never considered,
Speaker 1: then the photon could be turning into them, could be
Speaker 1: like interacting with them, could be like popping into existence
Speaker 1: some weird new particle you never imagined. And that would
Speaker 1: change how it interacts with the muan because it would
Speaker 1: lose some energy. It would just it would change its angle,
Speaker 1: it would change its direction, it would change the probability
Speaker 1: of this thing happening at all. And so in this way,
Speaker 1: the photon interacting with the muan is sort of like
Speaker 1: a probe of the whole universe because along the way,
Speaker 1: the photon can do all sorts of crazy stuff. You
Speaker 1: can do anything that quantum mechanics lets it do, and
Speaker 1: what happened affects how it interacts with the muon. And
Speaker 1: so by calculating this quantity and then measuring it, you
Speaker 1: can ask, like, is there anything else that the photon
Speaker 1: is doing along the way that's changing how it interacts
Speaker 1: with the mu want to see, like how good are
Speaker 1: we predicting what photons actually do. Yeah, it's like you
Speaker 1: said to photons, hey, go crazy, do anything you want
Speaker 1: to do, and then we're going to try to calculate
Speaker 1: all the things we think you can do, and then
Speaker 1: let's compare. And you know, if it turns out you're
Speaker 1: dancing with a new kind of particle we've never heard
Speaker 1: about before, we're gonna know you're like stalker fans. Yeah,
Speaker 1: and you know, people like me, I like to discover
Speaker 1: new particles by sort of making them concretely, like pouring
Speaker 1: enough energy into a collider so that we have enough
Speaker 1: energy to make this new particle. And see it's sort
Speaker 1: of directly, but this is another way to do it,
Speaker 1: is to like look for these particles just sort of
Speaker 1: like briefly popping into existence as photons do their crazy
Speaker 1: dance with muans. And I guess my question is why
Speaker 1: the mean like couldn't I mean, all these questions and
Speaker 1: all these magnetic moment ideas we should work for any
Speaker 1: other particle, right, So why are we focusing on the
Speaker 1: muon specifically? And you can do these calculations also for
Speaker 1: the electron and also for the town, right, but the
Speaker 1: muon is sort of in a sweet spot because it's
Speaker 1: a little bit heavier, it's sort of easier to handle.
Speaker 1: The new physics should happen to all of these particles. Right,
Speaker 1: but it has essentially a proportionately larger effect on the
Speaker 1: muan because it has a larger mass. I see, So
Speaker 1: lemon is like the guinea pig. Yeah, the muan is
Speaker 1: like the best place to get the universe to reveal
Speaker 1: all these little details, all right, And so you can
Speaker 1: run the math and it should tell you how the
Speaker 1: muon should bend in a magnetic field. And you can
Speaker 1: also measure how, like you can throw me on at
Speaker 1: a magnetic field and see how it bends. That's the
Speaker 1: experimental side. Yeah, But before we move on to the
Speaker 1: experimental side, I gotta sort of shout out to the
Speaker 1: theory here, because this is what I meant earlier about
Speaker 1: being really precise. On the theoretical side, this quantity, the
Speaker 1: magnetic moment of the muan, is the number that theorists
Speaker 1: know best. It's the most precisely calculated quantity basically in
Speaker 1: the universe as far as we know, unless there are
Speaker 1: alien physicists doing it out there. What how can something
Speaker 1: theoretical be precise? Doesn't precision mean like how right you are.
Speaker 1: It doesn't mean how right you are. And when we
Speaker 1: do these calculations, we start with the simplest idea as
Speaker 1: we say, well, what's the simplest thing of photon can do?
Speaker 1: And that gets you mostly right, and they think, well,
Speaker 1: what if it does one weird thing along the way,
Speaker 1: And there's like nineteen ways for that to happen, So
Speaker 1: you add nineteen calculations. Well what if it did two
Speaker 1: weird things along the way. Okay, now there's nineteen squared
Speaker 1: ways to do that, and each of these gives us
Speaker 1: smaller and smaller effect. And so as you add up
Speaker 1: more and more of these ideas you're considering, you get
Speaker 1: closer to the true answer, but also becomes harder. And
Speaker 1: so now they're at the point where they're calculating like
Speaker 1: millions and millions of possibilities. Maybe first to turn to
Speaker 1: ano electron, and that electron did some weird thing, which
Speaker 1: turn to into a photon, which then did some weird thing.
Speaker 1: And so they've estimated sort of theoretically how precise this is,
Speaker 1: Like it's impossible to get it exactly right because you
Speaker 1: need to do an infinite number of calculations, so they
Speaker 1: can estimate how close they get based on how much
Speaker 1: is the answer changing as they add more ideas, So
Speaker 1: they're asthm topically approaching the deep truth. I guess there's
Speaker 1: a you know, an engineering, there's always this issue about
Speaker 1: the different between accuracy and precision, like accuracies how right
Speaker 1: you are, and precision is like how sure you are?
Speaker 1: So is the thing that's happening here? Is it that
Speaker 1: theories are pretty sure they know what the moment of
Speaker 1: the muan is, like they think they've covered all the angles,
Speaker 1: so they're pretty sure, but maybe they don't know if
Speaker 1: it's the actual value. Yeah, you know, I have equivalent
Speaker 1: with theoretical physics here because experimentalists trying to be really
Speaker 1: formal about the statistical statements we make. If we say, okay,
Speaker 1: there's an uncertainty here, that means something very specific. Statistically,
Speaker 1: it means if you did the same experiment a hundred times,
Speaker 1: you would get the answer within your uncertainty bounds sixty
Speaker 1: percent of the time or something like that, or a
Speaker 1: different answer if your Bayesian. That's precision. There is are
Speaker 1: a lot more hand wavy, you know, they're like, well,
Speaker 1: we tweaked a couple knobs and got different answers, and
Speaker 1: so you know that's the uncertainty. We multiplied some things
Speaker 1: by two just to see how things would change. So
Speaker 1: that's what we're calling the uncertainty. And you know, it's harder,
Speaker 1: it's different. They're not measuring things about the universe. They're
Speaker 1: just trying to like guess how closely they are the
Speaker 1: right answers. So I guess maybe the title should really
Speaker 1: be the most precisely guessed that theory quantity ever, you
Speaker 1: know what I mean. Like they put a lot of
Speaker 1: attention into the They've covered every angle on so they're
Speaker 1: they're pretty sure that this is what the plan is. Yeah,
Speaker 1: I suppose so, although you know, there have been moments
Speaker 1: in this history and this is a decades long project
Speaker 1: to make the theory more precise and make the experiment
Speaker 1: more precise. It's a bit of an arms race to
Speaker 1: see like who's getting more and more precise. There was
Speaker 1: a moment in the nineties when the theorists discovered that
Speaker 1: they had gotten the sign wrong, like that a minus
Speaker 1: sign where there should be a positive sign, and it
Speaker 1: changed the answer kind of a lot. So they're they're
Speaker 1: definitely mistakes in there. Oh my gosh, who made the mistake?
Speaker 1: Are they going to put that in their tombstone as well?
Speaker 1: One more minus sign? Know, there are different groups, and
Speaker 1: they're cross checking each other, and so you know, that's
Speaker 1: another way they try to estimate how correct these things.
Speaker 1: All right, well, let's get in to now the experiment
Speaker 1: part of it and how well these two things match up.
Speaker 1: Who's more precise or less accurate or more marvelous. But
Speaker 1: first let's take a quick break. All right, we're talking
Speaker 1: about the magnetic moment of the muan as the most
Speaker 1: precisely guest at quantity ever, and now we're going to
Speaker 1: measure it with an experiment, and that just involves throwing
Speaker 1: a muan at a magnetic field and seeing where it
Speaker 1: goes or is there is there something special going on?
Speaker 1: You know, that would work, But what you want is
Speaker 1: a really precise measurement. You want a measurement which is
Speaker 1: accurate to like one part in ten to the twelve
Speaker 1: or ten to the thirteen, and so to do that
Speaker 1: you need a really clean setup. And so what you
Speaker 1: described would work, but it's sort of hard to measure
Speaker 1: it's a single particle. And so what you want is
Speaker 1: a lot of muans. You want them all basically doing
Speaker 1: the same thing. So you can get a bunch of
Speaker 1: measurements and divide by a big number and it sort
Speaker 1: of averages out some of the mistakes. And so what
Speaker 1: they do is they get a huge pile of muans,
Speaker 1: a big blob of muans, and they point the spin
Speaker 1: of the muan, which is the thing that determines again
Speaker 1: where this magnetic field is going, and they get them
Speaker 1: to spin in the direction they're moving, and they move
Speaker 1: them in a circle. So they have this ring in
Speaker 1: Chicago where they have a bunch of muans and they
Speaker 1: move them in a circle. And when muans move around
Speaker 1: in a circle in a magnetic field, their spin will precess,
Speaker 1: it will rotate around the access of motion. Because that's
Speaker 1: how the physics work out. Like if you try to
Speaker 1: ban the muon, it will also sort of change in
Speaker 1: other ways. Yeah, Like one thing that happens to a
Speaker 1: particle and you put it through a magnetic field is
Speaker 1: that it bends. But a particles moving in a circle
Speaker 1: through a magnetic field will process, It will will change
Speaker 1: the direction in which they're pointing. So that's what they
Speaker 1: can do, is they can measure the difference between the
Speaker 1: direction of the magnetic field that they're putting on these
Speaker 1: particles and the direction of the spin of the ones
Speaker 1: which affects their magnetic moment, And so they have come
Speaker 1: up with really clever ways to measure these things and
Speaker 1: to reduce all sorts of uncertainties. And you know, if
Speaker 1: you're a visual person, it's really very similar in spirit
Speaker 1: to the experiment that looks for gravitational waves. What you're
Speaker 1: trying to do is isolate this experiment from any other effect.
Speaker 1: You know, like, is that the microwave oven in the
Speaker 1: break room that's changing the answer? You do we understand
Speaker 1: all the electromagnetic fields nearby. Is the radiation from the
Speaker 1: ground affecting our result? It's this kind of experiment you're
Speaker 1: like really isolating any source of noise or uncertainty. All right,
Speaker 1: So they're spinning these muans in a circle in Chicago,
Speaker 1: and again not in Minnesota or Milwaukee or Montana. No,
Speaker 1: it's being done at Fermulab, the accelerator complex just outside
Speaker 1: Chicago between Batavia and Naperville, where I did my pH
Speaker 1: d thesis hometown plug. All right. So they're spinning these
Speaker 1: in a in a circle, and they're measuring how they're
Speaker 1: processing or changing in the direction of their moment, and
Speaker 1: that tells you the magnetic moment of the muan experimentally,
Speaker 1: and now the problem is how well does it match
Speaker 1: with what the theorists. Yeah, that's right, that's the questions.
Speaker 1: That we have the number from the theory and the
Speaker 1: number from the experiment, and if you write these two
Speaker 1: numbers down on a piece of paper, they agree to
Speaker 1: the first what is it, like eight or nine digits
Speaker 1: before they disagree. So it's like it's really a testament
Speaker 1: to an incredible amount of work. I mean, you call
Speaker 1: it guessing, but like these theorists have done a huge
Speaker 1: amount of work to really nail this down, and the
Speaker 1: experimentalists have done a different, difficult pile of work, and
Speaker 1: now they have these two numbers. It's incredible to me
Speaker 1: that they agree this closely at all. All right, so
Speaker 1: let's maybe sign out the number for the audience here.
Speaker 1: So the experimentalists say that the magnetic moment of the
Speaker 1: muan is two point zero zero two three three one
Speaker 1: eight four one eight close our mind is some small quantity,
Speaker 1: and what are the units of these These are dimensionless units. Yeah,
Speaker 1: so okay, that was from the experimentalist. Theorists say it
Speaker 1: should be two point zero zero two three, three, one
Speaker 1: eight three six to not fo, that's right. So they
Speaker 1: agree on you know, after the decimal place, they agree
Speaker 1: to seven digits, and then they disagree. One of them
Speaker 1: says four one, eight and the other one says three
Speaker 1: six two, which is not a huge difference. It's like
Speaker 1: twelve zeros and then like, yeah, it's a bunch of
Speaker 1: zeros and fifty six. But the fascinating thing is that
Speaker 1: both of them are pretty confident in their results. So
Speaker 1: there's a gap between them, very tiny gap between them,
Speaker 1: but the uncertainty is smaller than the gap. Right, the
Speaker 1: difference between them is fifty six, and the uncertainty is
Speaker 1: like fifteen, So the difference is like three or three
Speaker 1: and a half times the uncertainty. It seems really it's
Speaker 1: so weird to me that they're so confident, you know
Speaker 1: about these numbers, Like you know, I've done experiments, and
Speaker 1: you know, to get that kind of position is really hard,
Speaker 1: Like if they ran this experiment next year and the
Speaker 1: year after that, but they still get the same exact. Yeah.
Speaker 1: These uncertainties reflect statistical limitations, so like you haven't runned
Speaker 1: for an infinitely long time, and also systematic uncertainties like
Speaker 1: things you think will contribute to mismeasurement or or bias
Speaker 1: on your result, and you know these are estimates. It
Speaker 1: could be that they're wrong. It could be just a
Speaker 1: basic mistake somewhere. But this is what we're trying to learn.
Speaker 1: Like we're trying to learn, like do we understand how
Speaker 1: to do these precision measurements or do we understand how
Speaker 1: to do these calculations, or is there a new particle
Speaker 1: out there that we're not factoring into our calculations that's
Speaker 1: playing with a magnetic moment of the muant finding a
Speaker 1: little bit? Is this the hint of the discovery of
Speaker 1: some new particles, some new supersymmetric particle which is too
Speaker 1: heavy to make a particle colliders and only appears very
Speaker 1: briefly and gives these little hints to the mule, like
Speaker 1: is there something hiding in that zero point zero zero
Speaker 1: five six difference between the experiment and the theory, or
Speaker 1: because they're both pretty sure of their numbers, there's now
Speaker 1: like they're both pretty sure of their numbers. Yeah, it
Speaker 1: couldn't be like a wire missing here or a plus
Speaker 1: sign missing over there. They certainly could be and their
Speaker 1: independent checks, their independent experiments, and we'll talk about that
Speaker 1: in a moment, but they're both pretty confident. And I
Speaker 1: remember learning about this in college and I was still
Speaker 1: learning about quantum mechanics and how it all worked. And
Speaker 1: at the time, I thought of physics as sort of
Speaker 1: like a description of what we see about the universe,
Speaker 1: just like sort of a human internal to our minds
Speaker 1: approximation of what's happening in the universe. And I read
Speaker 1: about this calculation, like, wow, it agrees to you know,
Speaker 1: nine or ten decimal places. That's amazing. And I at
Speaker 1: this moment where I thought, wait a second, maybe physics
Speaker 1: isn't just describing approximately what's happening. Maybe we've discovered like
Speaker 1: the source code, Like maybe this is what the universe
Speaker 1: itself is doing. Because to get that accurate, to get
Speaker 1: that precise, it's sort of shocking, you know, to imagine
Speaker 1: there could be another theory that could also be that precise.
Speaker 1: So I see, it's like, what if we actually uncover
Speaker 1: at the code of the simulation of the universe, because
Speaker 1: it's so we're so right, and we're so right. Yeah,
Speaker 1: maybe the universe does run on a computer using these equations?
Speaker 1: Is that kind of what you mean? Sort of you know,
Speaker 1: but in a more universal way, like maybe the universe
Speaker 1: does follow laws and it does calculations, and it follows
Speaker 1: these rules when it does those calculations, you don't have
Speaker 1: to be embedded in some meta universe and simulated on
Speaker 1: a computer. Maybe the universe is doing calculations though. Anyway,
Speaker 1: it's an incredible testament in my mind to the work
Speaker 1: involved here, and it's amazing that it works at all.
Speaker 1: I agree, right, But there is sort of an interesting mystery,
Speaker 1: And I guess the weird thing is that you're telling
Speaker 1: me that for the electron there's no difference between the
Speaker 1: experiment and the theory. That's right, Like this difference only
Speaker 1: shows up in the melee. We can do the same
Speaker 1: measurement for the electron. We actually a similar number, but
Speaker 1: there's no discrepancy. Like the electron. When they do the
Speaker 1: theoretical calculation and they do the experimental measurement, they get
Speaker 1: those two things to agree to within uncertainty. Now we
Speaker 1: expect that new is it, new particles whatever, would have
Speaker 1: a bigger effect on the muan, so it's not a
Speaker 1: surprise that it doesn't appear there for the electrons. And
Speaker 1: that's quite fascinating. So maybe there's something going on with
Speaker 1: the muan that you wouldn't see in the electron. So
Speaker 1: the electron you check that box or like the theory,
Speaker 1: and both groups have gone at the electron with the
Speaker 1: same kind of intensity and precision, and you can do
Speaker 1: all the same kinds of theoretical calculations for the electron
Speaker 1: and get a really precise number. And then you can
Speaker 1: go measure the magnetic moment of the electron, because electrons
Speaker 1: also bend in magnetic fields, and you can make that
Speaker 1: measurement really, really precise, and those two numbers agree. Electrons,
Speaker 1: we understand them like. There are no mysteries hiding under
Speaker 1: the rug for the magnetic moment of the electron, but
Speaker 1: for the muan, which is exactly where we would expect
Speaker 1: to see something weird. First we start to see something weird,
Speaker 1: all right, But it's different for the muan, which means
Speaker 1: that it might be hiding a secret. So what does
Speaker 1: that mean, Daniel, What could be hiding underneath the marvelousness
Speaker 1: of the muant? Well, you know, we suspect that there
Speaker 1: are other particles out there that we have not yet discovered.
Speaker 1: We found six particles that are corks, six particles that
Speaker 1: are leftons, and then a few of the particles that
Speaker 1: mediate the interactions between them, and so we have this
Speaker 1: pile of particles, but we don't know if those the
Speaker 1: only particles out there. And actually it would make a
Speaker 1: lot more sense if there were more particles, because there
Speaker 1: are these weird patterns we found that are unexplained, and
Speaker 1: some of them would click together really nicely if there
Speaker 1: were new particles. Like some of the particles we've seen
Speaker 1: are called fermions. They have spin one half, and the
Speaker 1: other ones are called bosons because they have spin one.
Speaker 1: There's one idea that maybe every fermion has a boson version,
Speaker 1: like the muan has another version of it called the
Speaker 1: smeu on, and the photon is another version of it
Speaker 1: called the photino. And these are like just one idea
Speaker 1: of how there could be new particles out there that
Speaker 1: sort of solve deep problems in theoretical physics, but we
Speaker 1: haven't seen them yet, so they could just be too big,
Speaker 1: too heavy for us to discover them. In particle colliders,
Speaker 1: remember to see something in the collider, you have to
Speaker 1: put in enough energy, which means you have to make
Speaker 1: the collider big enough, which means you have to get
Speaker 1: enough money from the government to build a really big tunnel,
Speaker 1: So there's a limitation there. This might be another way
Speaker 1: to like sneak around that limitation and see these new
Speaker 1: particles for the first time, at least hint that they're there.
Speaker 1: And if you do the calculations and what do you
Speaker 1: expect to see if there are these new particles, this
Speaker 1: is kind of exactly what you expect to see. So
Speaker 1: is the idea then that maybe there's a new part
Speaker 1: that we don't know about that the photon is turning
Speaker 1: into or like transforming into before it interacts with me.
Speaker 1: Because the photon can interact with anything that has electric charge.
Speaker 1: And so if there's some new heavy particle out there
Speaker 1: that does have electric charge but it's never really exists
Speaker 1: in the universe because it's too massive, well, occasionally the
Speaker 1: photon can turn into it or pairs of it, like
Speaker 1: it's particle and it's antiparticle, and that would change how
Speaker 1: it interacts with the muan because you have to include
Speaker 1: it in all of these calculations. Like maybe it admits
Speaker 1: this new heavy particle and then it interacts with the
Speaker 1: muan and then it reabsorbs that particle, and that would
Speaker 1: change the way it interacts with the muan. And so
Speaker 1: the presence of weird heavy particles changes the basic interaction
Speaker 1: between two very simple particles, which I think is fascinating.
Speaker 1: It's like a it's a clever way to leverage you
Speaker 1: know something about the universe, to force the universe to
Speaker 1: tell you about what's going on, even if you don't
Speaker 1: have the energy to build that com eider. I think Danny,
Speaker 1: what you're saying is that the experimentalist are right and
Speaker 1: the theories are wrong. Well, you know, the experimentalists are
Speaker 1: probably wrong in different ways from the theorists. Experimentalists definitely
Speaker 1: make mistakes. It's really hard to do these things and
Speaker 1: to get them right and to remove all sources of air.
Speaker 1: And that's why it's fascinating as a cross check, because
Speaker 1: if they're wrong, they're probably wrong in different directions or
Speaker 1: different amounts. And so it's a great way to cross check,
Speaker 1: and you know, to improve experimental physics and to improve
Speaker 1: our theoretical understanding of the universe and maybe find new
Speaker 1: particles in between. And yeah, and maybe make us stop
Speaker 1: off in Stockholm to collect your Noobile prize. And that's
Speaker 1: why you know this isn't over. It's not just like, oh, hey,
Speaker 1: we saw this discripancy were done, because it's kind of indirect, right,
Speaker 1: This is not like we make these particles, we see them,
Speaker 1: we understand them. It's just sort of like a clue
Speaker 1: that the particles are there. And so what they want
Speaker 1: to do is make these things more precise. They want
Speaker 1: to get better experimental measurements. They want to push the
Speaker 1: theoretical measurements to see are these things wrong? Do these
Speaker 1: stack up? Can we improve this uncertainty? Can we make
Speaker 1: these things ten times as precise? And does it stick
Speaker 1: around or disappear? All? Right? Well, I think it all
Speaker 1: speaks to just again this idea that there may still
Speaker 1: be amazing things hiding even in tiny little gaps of
Speaker 1: point zero zero zero zero zero five six. That's right,
Speaker 1: some of the biggest clues in the universe turn out
Speaker 1: to be on the smallest numbers. And there is news
Speaker 1: to come because there's an experiment happening right now again
Speaker 1: in Chicago that's going to give a measurement of the
Speaker 1: muan magnetic moment that's going to be four times as
Speaker 1: precise as the one that we have now. They took
Speaker 1: this big magnet and they shipped it from Long Island
Speaker 1: where they did the experiment first, and moved into Chicago
Speaker 1: and that a cleaner beam with more muans and they're
Speaker 1: running those results right now. And in eighteen they said
Speaker 1: that they would have results quote sometime in t and
Speaker 1: so here we are in no results yet, but we
Speaker 1: expect any day, any day. These things are hard. We
Speaker 1: expect any day they'll come out with the new measurement.
Speaker 1: And the whole physics community is waiting, like, what's going
Speaker 1: to happen. How's the number going to change? So it's
Speaker 1: a big deal. They're like, hey, we said we'd be
Speaker 1: accurate about the muan, not about when we would tell
Speaker 1: you about the plus or minus five years, plus the
Speaker 1: minus fifty six years. Oh man, I'm sure that there
Speaker 1: are some graduate students out there on this experiment it's
Speaker 1: called muan G minus two, that they are sweating and
Speaker 1: working hard to get this number out. Well, hopefully we
Speaker 1: added a little bit more pressure because now I'm curious
Speaker 1: about what's gonna happen. We're all curious because this is
Speaker 1: how we learned about the universe. We corner it and
Speaker 1: force it to tell us what is the answer to
Speaker 1: this number? We think we know what it should be.
Speaker 1: Tell us what the real truth is, tells the universe,
Speaker 1: don't keep it to yourself. Experimental physics is basically a
Speaker 1: modern day oracle, right, We actually do get to ask questions,
Speaker 1: will be horrible, and it gives us answer and then
Speaker 1: it chops off your head or something something Greek and
Speaker 1: classical like that. It kills your mom. Probably all right, Well,
Speaker 1: we hope you enjoyed that, and think about all the
Speaker 1: amazing secrets that could be hiding in the smallest of quantities.
Speaker 1: That's right. In one of these days, one of these
Speaker 1: secrets will reveal something deep and true about the universe.
Speaker 1: See you next time. Thanks for listening, and remember that
Speaker 1: Daniel and Jorge Explain the Universe is a production of
Speaker 1: I Heart Radio. Or more podcast from my Heart Radio
Speaker 1: visit the i heart Radio app, Apple Podcasts, or wherever
Speaker 1: you listen to your favorite